1. Field of the Invention
This invention is concerned with a process for the steam passivation of metal contaminants on cracking catalysts.
2. Prior Art
Before the introduction of sieve cracking catalysts in the early 1960's, there was great interest in processes for demetallization. The onset of the use of sieve catalysts with their higher tolerance for metals than nonsieve cracking catalysts temporarily reduced the interest in demetallization.
With the present growing demand for gasoline and light fuel oil, coupled with the increasing cost and decreasing availability of crude oil, there is now a renewed interest in demetallization due to the need to refine heavier crude fractions. These heavier crude fractions, such as those produced by atmospheric and vacuum crude distillation columns, are generally characterized as being undesirable as feedstocks for many refinery processes due primarily to their high metals content.
Principal metal contaminants are nickel and vanadium, with iron and small quantities of copper sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. As the great majority of these metals when present in crude oil is associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all the metal present in the crude, such metals being particularly concentrated in the asphaltene residual fraction. The metal contaminants are typically large organometallic complexes such as metal porphyrins and asphaltenes.
When metals are present in a cracking unit chargestock, such metals are deposited on the cracking catalyst. The metals act as a catalyst poison and greatly decrease the efficiency of the cracking process.
Some of the deleterious effects of metal poisoning of cracking catalysts are as follows:
Reduced catalyst activity and selectivity; PA1 Large increases in coke yield; PA1 Large increases in hydrogen yield on a molar basis; PA1 Decrease in gasoline yield; PA1 Maxima of gasoline yield occur at lower conversion; PA1 Physical deterioration of catalysts; PA1 Catalysis of nonselective cracking of gas oils to undesirable products and catalysis of dehydrogenation and condensation reactions; and PA1 Decrease in propylene, butene and isobutane yields.
The conventional approach to control the metals level in cracking units is to increase the makeup rate of fresh catalyst, coupled with an increase in withdrawal of inventory catalyst in order to maintain a constant amount of catalyst in the inventory. The higher the metal content of the feedstock, the greater the makeup rate of fresh catalyst is required and the greater is the expense. Therefore the use of the conventional approach to feedstocks having high metal contents would be quite expensive.
Prior to the discovery and use of sieve cracking catalysts in the early 1960's, various processes were employed for the demetallization of fluid cracking catalysts of the amorphous silica-alumina (nonsieve) type. These processes can be classified into six general categories--Acid Resin Contact, Acid Complex, Inorganic Acid Treatment, Treatment with NH.sub.4 OH, Volatilization and Simple Abrasion.
Acid resin contact involves pretreatment of the metal contaminated catalyst with sulfuric acid, followed by oxidation and contact with an aqueous slurry of cationic resin. This general process is described in U.S. Pat. No. 3,123,548, with similar processes described in U.S. Pat. Nos. 3,192,151 and 3,260,676.
The acid complex process entails activating the contaminated catalysts with oxygen, hydrogen sulfide and chlorine, followed by contact with aqueous acid complexing agents such as citric acid. This process is described in U.S. Pat. No. 3,122,511.
Inorganic acid treatment involves contacting the contaminated catalysts with such acids as H.sub.2 SO.sub.4, HNO.sub.3, HCl or H.sub.2 SO.sub.3. The following U.S. Patents discuss this general process: U.S. Pat. Nos. 3,037,882; 3,122,497; 3,147,209; 3,147,228; 3,222,293; and 3,234,145.
Treatment with NH.sub.4 OH, as described in U.S. Pat. No. 3,150,103, involves precalcination of the metal contaminated catalysts and then contact with NH.sub.4 OH.
In the volatilization process, the metal contaminated catalyst is prereduced, then subjected to carbonyl formation with carbon monoxide interaction at low temperature under pressure. This process is described in U.S. Pat. Nos. 3,151,088 and 3,168,482.
In the simple abrasion process, the preoxidized metal poisoned catalyst is contacted with hydrogen sulfide gas at flow rates high enough to cause abrasion. This technique is disclosed in U.S. Pat. No. 3,151,059.
The aforementioned six general categories of processes were developed in conjunction with nonsieve cracking catalysts. The general applicability of all these processes to sieve (zeolite) cracking catalysts is not fully known. Many of the treatments involved in these processes may prove to be harmful when dealing with catalysts containing zeolites. Treatments at low pH will necessarily have to be eliminated, or of very short duration to avoid acid destruction of zeolite catalysts. At the same time, the metals must be in a readily soluble form to avoid the forming of complexes of these metals with the alumina in the zeolite. This may require some type of pretreatment prior to any solubilization step.
A recent process dealing with the chemical removal of metal poisons from equilibrium catalyst is described by Edison et al, Crude and Resid Can be Cat-Cracker Feeds, OIL & GAS JOURNAL, Dec. 20, 1976. This process is similar to the acid resin contact process and can be used with zeolite catalysts.
Recent developments dealing with the problem of metal poisoning of cracking catalysts involve the utilization of certain compounds to passivate said metals. U.S. Pat. Nos. 3,711,422; 4,025,458; and 4,031,002 describe the use of antimony compounds to passivate metals on a cracking catalyst. U.S. Pat. No. 3,977,963 discloses a method of negating the effects of metal poisoning on a cracking catalyst by the use of bismuth or manganese compounds.
It would be very advantageous to have a relatively inexpensive and nontoxic process to negate the deleterious effects of metal poisoning of cracking catalysts. Such a process is presented in this invention.